Generally, touchpad systems are implemented for a variety of applications. Some of these applications include, computer interfaces, keypads, keyboards, and other applications. Various types of touch pads are known. Optical touch pads have certain advantages over some other types of touch pads at least for some applications. Various types of optical touchpad systems may be used in some or all of these applications. However, conventional optical touchpad systems may include various drawbacks. For example, conventional optical touchpad systems may be costly, imprecise, temperamental, fragile, energy inefficient, or may have other weaknesses and/or drawbacks. Further, conventional systems may require that an interface surface (e.g. a surface that is engaged by a user) have a generally planar, or otherwise uniform topology. Some conventional optical touchpads may require that light reach the interface surface to enable an object in engagement with the interface surface to interact with the light. Thus some, conventional optical touchpad systems may require that the interface surface be either (i) predominantly transparent, or (ii) predominantly opaque. These limitations on topology and/or opacity may restrict the use of these systems with respect to some of the applications for touchpad systems. Various other drawbacks exist with known touchpads, including optical touchpads.